Stimulus-Responsive Water-Soluble Polymers Based on 2-Hydroxyethyl Methacrylate

Abstract

The homopolymerization and block/statistical copolymerization of 2-hydroxyethyl methacrylate (HEMA) using atom transfer radical polymerization (ATRP) in methanol at 20 °C has been investigated. For the homopolymerizations, both high conversions and low polydispersities (Mw/Mn < 1.25) were obtained over a wide range of target degrees of polymerization. According to the literature, HEMA homopolymer is usually described as only water-swellable, but in this work low molecular weight HEMA oligomers (target degrees of polymerization, DPn, less than 20) exhibited water solubility over a wide temperature range (no cloud point behavior). Furthermore, for actual DPn's between 20 and 45, HEMA homopolymers exhibited inverse temperature solubility in dilute aqueous solution at pH 6.5, and their cloud points increased systematically as the DPn was reduced. Gravimetric studies indicated that “water-insoluble” HEMA homopolymers with DPn's higher than 50 were actually partially soluble: GPC studies confirmed that fractionation occurred due to preferential dissolution of the shorter chains. Furthermore, HEMA homopolymers with DPn's up to 50 are water-soluble at pH 2.2 and do not exhibit cloud points. This is attributed to protonation of the terminal morpholine groups derived from the ATRP initiator. Thus, depending on the mean DPn and the solution pH, water can be a good solvent, a marginal solvent or a nonsolvent for HEMA homopolymer. Chain extension (self-blocking) experiments conducted for the ATRP of HEMA in methanol at 20 °C using a Cu(I)Cl catalyst and bpy ligand indicated reasonable living character. Statistical copolymerizations of HEMA with other comonomers such as glycerol monomethacrylate (GMA) and 2-hydroxypropyl methacrylate (HPMA) allowed the cloud point behavior to be manipulated. Finally, a range of novel HEMA-based block copolymers were synthesized in which the HEMA block was either thermoresponsive or permanently hydrophilic, depending on its DPn and the nature of the second block. Thus, diblock copolymer micelles with either hydroxylated cores or coronas could be prepared.